The present invention relates to the field of microelectronic devices, and, more particularly, to an inhibitor layer for inhibiting moisture-generated corrosion of aluminum (Al) alloy metallization layers in microelectronic devices and to a method of fabricating such a layer for semiconductor integrated electronic devices.
As is well known in the art, corrosion of a final metallization layer typically occurs when electronic devices are assembled and encapsulated in plastic packages. Such corrosion presents a serious problem of reliability in the manufacture of semiconductor integrated electronic devices. For example, the performance of power devices incorporating metal oxide semiconductor (MOS) power transistors is deeply affected by corrosion of the metal layer on the device surface. This effect becomes more acute as the dimensions of integrated circuits grow smaller.
A determining value for satisfactory operation of a semiconductor device is its resistance in the power-on condition. This resistance value can be significantly altered by oxidation of the metal layer on the device surface. In power devices, the metal layer is coincident with the source electrode of the power MOS. Surface metallization is common practice in the semiconductor industry and involves depositing a layer of an aluminum alloy (typically Alxe2x80x94Si (1% w/o)) in semiconductor devices. Electro-migration in semiconductor devices is of less concern than in power switches using power MOS.
The corrosive phenomenon in a damp environment can start at the surface of the Alxe2x80x94Si alloy whenever condensation collects thereon. This is because water contains a sufficient concentration of corrosive ion impurities dissolved therein. Water from condensation can also form locally on the metal layer surface, e.g., as a result of water penetrating the package and the molding protection resin during tests for operability in moisture. Such operability tests are occasionally carried out inside pressure cookers.
Certain ion species flowing between the cathodic and the anodic surface regions may be released from the molding resin itself. They may also be contaminants in the materials used for making the integrated circuit. The molding resin, further to absorbing and possibly becoming saturated with water, can act as an electrolytic medium. Yet, the diffusion of ions and electrons through the resin layer makes it difficult to retard and ultimately check the corrosive processes.
It should also be noted that an electrolytic cell may form on the alloy layer surface. Its cathode and anode usually are two electrodes of the device (e.g., the gate and source metal contacts in a MOS power transistor) or may be the gold bonding wire and the surrounding surface of the Al metallization layer.
The Al metallization layer is normally coated with a very thin (a few tens) of Angstroms layer of native oxide hydrate which preserves the metal beneath from further corrosion in standard environmental conditions. However, this native layer becomes eroded where the condensed electrolyte has a sufficient amount of catalytic agents. In this case, the Al oxide hydrate is dissolved by chemical reaction. This results in the surface of the Al metal layer being rapidly brought to contact with the electrolyte. Furthermore, a galvanic corrosion mechanism is initiated in the Al layer through one of two possible reaction paths, according to the local pH value, as follows:
in an acidic medium,
2Al+6H+xe2x86x922Al3++3H2
2Al3++6H2Oxe2x86x922Al(OH)3+6H+
and in a basic medium,
Al+3OHxe2x86x922Al(OH)3+3exe2x88x92
6O2+6H2O+12exe2x88x92xe2x86x9212OHxe2x88x92
A first prior approach to making corrosion less likely to occur is based on forming a suitable inhibitor layer over the surface of the aluminum alloy layer. For this inhibitor layer to be effective, the layer and the processes for forming it must meet certain requirements. First, it must be consistent with the performance of the finished device. The chemical-physical characteristics of the other layers included in the integrated circuit should also be unaffected. Additionally, the inhibitor layers should be compatible with the subsequent steps of bonding the lead wires and packaging.
In plastic packaged devices, the aluminum metal layer surface is generally protected with a relatively thick layer of a passivation dielectric (e.g., SiN, Pvapox, SiON, etc.) which prevents water from migrating to the aluminum surface from the molding resin. In any event, the passivation dielectric must be conformed (e.g., by a photolithographic process and associated etch) to bond wires to the surface of the Al metal layer and produce the contacts.
In power devices where large currents are involved, bonding wires of diameters in the 2 to 20 mils range are typically used and require a relatively broad bonding area. The bonding ends represent uncovered metal regions and, accordingly, are potentially subject to the corrosive action of water in either saturated steam or liquid form. It can be appreciated, therefore, that the passivation dielectric ordinarily used cannot solve the above problem. Further process steps (e.g., dielectric depositing, masking, etching) are required which can introduce further problems from interaction with the underlying layers, as well as added cost.
The technical problem underlying this invention is to provide a protection or passivation for the metal layers present in semiconductor integrated circuits which exhibits appropriate structural and functional features to effectively protect the metal layer against corrosion. Such passivation should be provided without burdening the electronic device manufacture with additional complexity and cost. In this way, the aforementioned drawbacks of the prior art can be reduced.
The concept behind this invention is one of growing, over the metal layer, a very thin passivation phosphate layer or film to resist corrosion/hydroxidation of the metal when the latter is subjected to stresses in a damp environment. Preferably, this thin film is grown by chemical treatment of the Al alloy metal layer surface.
According to the invention, a semiconductor device includes at least one aluminum alloy metallization layer, a native aluminum oxide hydrate layer on the at least one aluminum alloy metallization layer, and a protective inhibitor layer on the native aluminum oxide hydrate layer. The protective inhibitor layer may include phosphate and it reduces moisture-generated corrosion of the at least one aluminum alloy metallization layer.
More particularly, the protective inhibitor layer may include a mixture of ortho- and poly-phosphate grafted phases adjacent the native aluminum oxide hydrate layer. The protective inhibitor layer may be less than about 50 xc3x85 thick, and, more preferably, less than about 35 xc3x85 thick. The native aluminum oxide hydrate layer may be less than about 50 xc3x85 thick, and, more preferably, in a range of about 30 to 40 xc3x85. Furthermore, the protective inhibitor layer may include aluminum phosphate and it may also be cured.
A method according to the invention of producing a protective inhibitor layer for reducing moisture-generated corrosion of aluminum alloy metallization layers in semiconductor devices includes chemically treating an aluminum alloy metallization layer at less than about 50xc2x0 C. using concentrated nitric acid formed in a substantially non-aqueous solution. The method may also include chemically treating the aluminum alloy metallization layer at less than about 50xc2x0 C. using a mixture comprising nitric acid and trace phosphoric acid formed in a substantially non-aqueous solution.